1. Field
The present invention relates generally to optical transmission systems, and more specifically to detecting and controlling light propagation in optical transmission systems.
2. Background
Optical technology has long been used to provide point-to-point connections in high-speed communication networks. The capacity of these links has increased dramatically in recent years with the development of dense-wavelength-division-multiplexed (DWDM) systems, wherein multiple signals are transmitted simultaneously on different wavelength channels. Switching and routing have traditionally been performed by converting the optical signals to electronic signals, employing electronic switches, and then converting the signals back into the optical domain. As optical technology becomes more mature, some routing functions are starting to be performed in the optical domain. Products are available for the metropolitan market that use optical-add-drop multiplexers (OADMs) to route different wavelengths to different destinations allowing some wavelengths to bypass nodes in a DWDM network. To make a communication network more flexible, it is desirable to be able to configure dynamically the network. Tunable OADMs would allow a network operator to change which wavelengths bypass nodes and which wavelengths are dropped.
OADMs are frequently made of optical circulators and wavelength-selective devices, such as Bragg gratings. Bragg gratings act as wavelength-selective mirrors. To make a network dynamically reconfigurable, tunable components, such as tunable filters or tunable lasers, can be used. Adding and dropping a subset of the wavelengths transmitted on a fiber can create problems in the transmission if a wavelength is not entirely dropped or if the add mechanism operates less than optimally such that some portion of the energy of the added signal propagates in an undesired direction, for example. This mixing of wavelengths creates a mixing of signals called crosstalk, which is typically an undesirable phenomenon.
To combat this problem and prevent unwanted signal mixing, optical isolators can be used to isolate a drop device or mechanism from an add device or mechanism. An example configuration is depicted in FIG. 1. In this known configuration, an optical isolator 106 is placed between an optical drop mechanism 104 and an optical add mechanism 108. This configuration ideally prevents any wavelengths added by optical add mechanism 108 from propagating in a direction A towards optical drop mechanism 104, because optical isolator 106 only allows signal flow in the direction indicated by its associated arrow. Without optical isolator 106 light added at ADD mechanism 108 may mix with light dropped at DROP mechanism 104, degrading the quality of the dropped signal.
While this arrangement is effective in blocking the added wavelength energy from mixing with or affecting the drop mechanism, it does nothing to improve the effectiveness or efficiency of the add operation itself. It would be useful to have a technique that not only blocks components from propagating in an undesired direction, but also provides a measurement of the level of effectiveness of the add operation itself. That measurement might then be used in a variety of ways.
Therefore, a need exists for an improved system and method for detecting and controlling wavelength propagation within optical transmission systems.